Asymmetric paternal effect on offspring size linked to parent-of-origin expression of an insulin-like growth factor.

Sexual reproduction brings together reproductive partners whose long-term interests often differ, raising the possibility of conflict over their reproductive investment. Males that enhance maternal investment in their offspring gain fitness benefits, even if this compromises future reproductive investment by iteroparous females. When the conflict occurs at a genomic level, it may be uncovered by crossing divergent populations, as a mismatch in the coevolved patterns of paternal manipulation and maternal resistance may generate asymmetric embryonic growth. We report such an asymmetry in reciprocal crosses between populations of the fish Girardinichthys multiradiatus. We also show that a fragment of a gene which can influence embryonic growth (Insulin-Like Growth Factor 2; igf2) exhibits a parent-of-origin methylation pattern, where the maternally inherited igf2 allele has much more 5' cytosine methylation than the paternally inherited allele. Our findings suggest that male manipulation of maternal investment may have evolved in fish, while the parent-of-origin methylation pattern appears to be a potential candidate mechanism modulating this antagonistic coevolution process. However, disruption of other coadaptive processes cannot be ruled out, as these can lead to similar effects as conflict.